Proposed studies have three components. The first is to continue the study of the efferent innervation of hair cells, supporting cells and spiral ganglion at the light and ultrastructural levels, characterizing neurotransmitters, efferent density and synaptology. These are following up and extending exciting findings during the current grant period which showed dendro-dendritic synapses in the outer spiral bundle, reciprocal synapses on outer hair cells, innervation of supporting cells and innervation of spiral ganglion cells. The second component is to confirm human findings and characterize the efferent types, using the guinea pig as a model where different efferent lesions can be made. The third component is an in depth examination of the co-pathology and hearing loss associated with the basilar membrane thickening in the human cochlea that they have observed with aging.